Electronic light-recording devices such as charge-coupled display (CCD) cameras, single element arrays, as found in InGaAs cameras, and so forth, have a dark response (i.e., a signal in the absence of light) which must be corrected. Normally this involves taking an exposure cycle in the absence of light from the sample to be measured and storing it as a “dark spectrum.” Light from the sample is then passed to the camera for an identical exposure cycle to generate an “uncorrected sample spectrum.” A “corrected” sample spectrum is then computed by subtracting the dark spectrum from the uncorrected sample spectrum. (Other forms of correction are then computed to correct for the spectral responsivity of the detectors, the spectral mapping of the array, interpolation, etc., but these are separate subjects outside the scope of this disclosure.) As the time between collection of the dark and the collection of light becomes larger, the dark data may not match the true camera response in the absence of light due to temperature fluctuation or other reasons.
If the dark spectrum is updated prior to each light exposure cycle, this essentially doubles the amount of time required for a total data collection cycle. Further, when analyzing spectra that contain both very weak and very strong spectral components of interest, the exposure cycle time required for adequate SNR (signal-to-noise)/quantitation on the very weak components, such as in analysis of gas mixtures by Raman spectroscopy, can be very long—on the order of several minutes. Stronger components in the same mixture may be accurately quantitated in a matter of seconds.
Previous attempts to solve the dark correction problem either are inefficient in the amount of time required, or inaccurate in matching the true dark response at the time of light collection. Existing techniques either collect one dark spectrum and apply it to all future spectra in an experiment or monitoring process, or collect a new dark spectrum before each signal spectrum.
Standard Practice 1
FIG. 1 illustrates current standard practice involving a single stored dark spectrum. A collection cycle consists of N accumulations of single exposures, each within the dynamic range of the array detector, summed or averaged to achieve a target SNR for the most difficult (typically the weakest) spectral feature in application. The resulting sum or average will henceforth be referred to as a spectrum A dark exposure is acquired with signal light blocked, thus acquiring one dark exposure at 102. A second dark exposure is acquired at 104 for cosmic event correction, and this process is repeated by summing or averaging the cosmic corrected exposures at 108 for N accumulations (106). In accordance with this disclosure, including the embodiments described here, “cosmic correction” should be taken to mean combining two (or more) exposures in such a way as to eliminate pixel signal if one of the exposures show evidence of cosmic ray spikes, while averaging the pixels from the both exposures if neither has a cosmic-ray-induced spike. Further, “N” is typically determined by the ratio of the strongest feature in the spectrum to the weakest feature, such that each of the N accumulations is sufficiently short to avoid detector saturation at the strongest feature, and the total exposure time T over N accumulations provides the required SNR for the weakest component.
The resulting dark spectrum is saved at 110 and subtracted at 112 from all subsequent signal collection spectra acquired in the same manner, but with signal light illuminating the detectors. The result is output at 114. This approach may comprise a standard practice for sufficiently stable dark current, which can be the case for very stable dark current, typically characterized by very stable thermal environments for both detector and spectrograph hardware. It can also be the case for applications with very strong signals relative to dark current. The cycle time for data within a run is the shortest possible because once the single dark spectrum is acquired, signal data is being acquired at all times. Total data reporting cycle time for the method of FIG. 1 is T, as dictated by the weakest component of interest.
Standard Practice 2
FIG. 2 illustrates an alternative standard practice involving interleaved dark spectra. A dark spectrum is acquired at 202, followed by the acquisition of signal spectra at 204. The dark signal is subtracted from the light spectrum at 206. A new dark spectrum is acquired over N accumulations as described above in between each signal cycle of N accumulations. This allows the instrument to correct for changes in dark current over the course of a data run. However, it doubles the data cycle time relative to Standard Practice 1, because half of the time is spent acquiring dark spectra, not signal. Thus, total data reporting cycle time is 2T.